Process control loop current verification

Abstract

Some embodiments are directed to a process device comprising a process variable sensor configured to generate an output signal indicative of a sensed process variable; loop current output circuitry configured to control a loop current on a two wire process control loop to a value based on the output signal; loop current measurement circuitry coupled to the process control loop and configured to generate a measured loop current value based on the loop current; and loop current verification circuitry configured to approximate the loop current value based on the output signal and properties of a low pass filter, and generate a diagnostic signal based on a comparison of the approximated loop current value and the measured loop current value.

What is claimed is: 
     
       1. A process device comprising:
 a process variable sensor configured to generate an output signal indicative of a sensed process variable; 
 loop current output circuitry configured to control a loop current on a two wire process control loop to a value based on the output signal processed by a first low pass filter wherein the first low pass filter introduces a first error in loop current in response to a step change in the sensed process variable; 
 loop current measurement circuitry coupled to the process control loop and configured to generate a measured loop current value based on the loop current, and as a function of a second low pass filter, wherein the second low pass filter introduces a second error in the measured loop current value in response to the step change in the sensed process variable; and 
 loop current verification circuitry configured to compensate for the first and second errors based on the measured loop current value and properties of the first and second low pass filters, and generate a diagnostic signal based on a comparison of an approximated loop current value and the measured loop current value. 
 
     
     
       2. The process device of  claim 1 , including:
 a digital-to-analog conversion circuitry configured to convert a digital measured value representative of the output signal to an analog measured value, the conversion circuitry including the low pass filter; 
 wherein the loop current output circuitry is configured to control the loop current as a function of the analog measured value; and 
 wherein the loop current verification circuitry is configured to approximate the loop current value based on properties of the first low pass filter. 
 
     
     
       3. The process device of  claim 2 , wherein the loop current verification circuitry is configured to approximate the loop current value based on a time constant of the low pass filter. 
     
     
       4. The device of  claim 1 , wherein the loop current verification circuitry is configured to approximate the loop current value based on a time constant of the second low pass filter. 
     
     
       5. The process device of  claim 4 , wherein the loop current verification circuitry is configured to recursively generate the approximated loop current value (I C2 (n)) based on the following equations:
     I   C1 ( n )= a   1   *I ( n )+ b   1   *I   C1 ( n− 1); and 
     I   C2 ( n )= a   2   *I   C1 ( n )+ b   2   *I   C2 ( n− 1); 
 wherein n represents the current sample, I C1  represents output from a first recursive filter, I C2  represents the approximate loop current value sample output from a second recursive filter, n−1 represents the preceding sample, I is the digital measured value, a 1  and b 1  are calculated based on a time constant of the first low pass filter, and a 2  and b 2  are calculated based on a time constant of the second low pass filter. 
 
     
     
       6. The device of  claim 1 , wherein the loop current verification circuitry comprises:
 a difference evaluator having an error estimate output corresponding to a difference between the measured loop current value and the approximated loop current value; and 
 a comparator configured to compare the error estimate output and a reference, and output the diagnostic signal responsive to the comparison. 
 
     
     
       7. The process device according to any of  claim 1 , wherein:
 the device comprises a digital-to-analog conversion circuit configured to convert a digital measured value representative of the output signal to an analog measured value; 
 the loop current output circuitry is configured to control the loop current as a function of the analog measured value; and 
 the loop current verification circuitry is configured to approximate the loop current value based on a correction factor stored in a memory. 
 
     
     
       8. The process device of  claim 7 , wherein the loop current verification circuitry is configured to approximate the loop current value based on the following equation:
     I   C ( n )= I ( n )− k*ΔI=I ( n )− k *( I ( n )− I ( n− 1));
 
 wherein I C  is the approximate loop current value, k is the correction factor, n represents the current sample, n−1 represents the preceding sample, and I is the digital measured value. 
 
     
     
       9. The process device according to  claim 1 , wherein the two wire process control loop is a 4-20 milliamp loop. 
     
     
       10. The process device according to  claim 1 , wherein the process device is powered by the two wire process control loop. 
     
     
       11. A method in a process device for verifying loop current in a two wire process control loop comprising:
 sensing a process variable; 
 changing a digital measured value from a first value to a second value as a function of the sensed process variable; 
 converting the digital measured value to an analog measured value using a first low pass filter; 
 adjusting a loop current in the two wire process control loop as a function of the analog measured value wherein the first low pass filter introduces a first error in loop current in response to a step change in the sensed process variable; 
 sensing the loop current in the two wire process control loop; 
 converting the sensed loop current to a measured loop current value using a second low pass filter wherein the second low pass filter introduces a second error in the measured loop current value in response to the step change in the sensed process variable; 
 compensating for the first and second errors based upon the measured loop current value and properties of the first and second low pass filters and generating for the first and second errors; and 
 generating a diagnostic signal based on a comparison of the approximated loop current value to the measured loop current value. 
 
     
     
       12. The method of  claim 11 , wherein approximating the measured loop current value comprises recursively generating the approximated loop current value (I C2 ) based on the following equations:
     I   C1 ( n )= a   1   *I ( n )+ b   1   *I   C1 ( n− 1); and 
     I   C2 ( n )= a   2   *I   C1 ( n )+ b   2   *I   C2 ( n− 1); 
 wherein n represents the current digital sample, n−1 represents the preceding digital sample, I is the digital measured value, a 1  and b 1  are calculated based on a time constant of the first low pass filter, and a 2  and b 2  are calculated based on a time constant of the second low pass filter. 
 
     
     
       13. The method of  claim 12 , further comprising retrieving the values for a 1 , b 1 , a 2  and b 2  from memory. 
     
     
       14. The method of  claim 11 , wherein approximating the measured loop current value comprises applying a correction factor that is based on properties of the first and second low pass filters to the digital measured value. 
     
     
       15. The method of  claim 14 , further comprising retrieving the correction factor from memory of the process device.